1. Field of the Invention
The present invention relates to a communication system, especially to an apparatus for appending cyclic redundancy check apparatus in the communication system.
2. Description of the Related Art
Now, 3GPP (the 3rd Generation Mobile Communication System Partnership Project) Standardization Organization has commenced on Long-term Evolution (referred to as LTE) to existing system criteria. Among numerous physical layer transmission techniques, both a downlink transmission technique based on OFDM (Orthogonal Frequency Division Multiplexing) and an uplink transmission technique based on SCFDMA (Single Carrier Frequency Division Multiple Access) are in hot research. In nature, OFDM is a multi-carrier modulation communication technique. Its basic principle is to divide a high rate data stream into multiple low rate data streams to transmit via a group of orthogonal sub-carriers simultaneously. Because of the nature of multi-carrier, the OFDM technique bears superior performance in many aspects. SCFDMA is essentially a single carrier transmission technique with comparatively lower PAPR (Peak to Average Power Ratio). Therefore, the power amplifier of a mobile terminal can be operated effectively to enlarge the cell coverage. In addition, with the adoption of cyclic prefix and frequency domain filtering, SCFDMA technique bears comparatively lower processing complexity.
The cyclic redundancy check (CRC) is a hash function for generating a few fixed number of data bits according to data such as network data packets or computer file bock. It is adopted to detect possible error for data transmission or data storage. CRC is calculated before the data transmission or data storage and is appended at the end of the data. And in a receiver, the data is checked whether it is changed or not.
One CRC calculation is as follows. Suppose sequence a0, a1, a2, a3, . . . , aA-1 is input into a CRC calculation module and a generated check-bit sequence is p0, p1, p2, p3, . . . , pL-1, where A indicates a length of the input sequence, and L indicates a length of the check-bit sequence. Then, a sequence appended with check bits is a0, a1, a2, a3, . . . , aA-1, p0, p1, p2, p3, . . . , pL-1. The check bits are calculated as follows: in GF(2), a polynomial expression
a0DA+L−1+a1DA+L−2+ . . . +aA-1DL+p0DL−1+p1DL−2+ . . . +pL-2D1+pL-1 is divided by corresponding generation polynomial, and a remainder must be zero.
At present, the CRC generation polynomials applied in LTE are as follows: if the length of CRC L=16, the CRC generation polynomial gCRC16(D)=D16+D12+D5+1; if the length of CRC L=24, the CRC generation polynomials are gCRC24A(D)=D24+D23+D18+D17+D14+D11+D10+D7+D6+D5+D4+D3+D+1 and gCRC24B(D)=D24+D23+D6+D5+D+1.
In current LTE, a DCI processing flow is illustrated in FIG. 1. In module 101, DCI adds the CRC in the data sequence. Suppose load information for PDCCH is a0, a1, a2, a3, . . . , aA-1. The check-bit sequence generated according to the CRC generation polynomial is p0, p1, p2, p3, . . . , pL-1, where A indicates the length of the load information and L indicates the length of the check-bit sequence. Suppose the sequence appended with CRC is b0, b1, b2, b3, . . . , bB-1, where B=A+L. Then the relationships between ak, bk and pk are as follows:
                                             b            k                    =                      a            k                                                            k            =            0                    ,          1          ,          2          ,          …          ⁢                                          ,                      A            -            1                                                                    b            k                    =                      p                          k              -              A                                                                        k            =            A                    ,                      A            +            1                    ,                      A            +            2                    ,          …          ⁢                                          ,                      A            +            L            -            1                              
After appending the CRC, a scrambling process is performed on the CRC check-bit sequence with the user equipment (UE) ID sequence xue,0, xue,1, . . . , xue,15 to form a sequence c0, c1, c2, c3, . . . , cB-1. The relationship between bk and ck is as follows:
                                             c            k                    =                      b            k                                                            k            =            0                    ,          1          ,          2          ,          …          ⁢                                          ,                      A            -            1                                                                                  c              k                        =                                          (                                                      b                    k                                    +                                      x                                          ue                      ,                                                                                          ⁢                                              k                        -                        A                                                                                            )                            ⁢              mod              ⁢                                                          ⁢              2                                ⁢                                                                                  k            =            A                    ,                      A            +            1                    ,                                          ⁢                      A            +            2                    ,          …          ⁢                                          ,                      A            +            15                              
A channel coding is performed on the sequence c0, c1, c2, c3, . . . , cB-1 in module 102. In LTE, a convolution coding scheme is applied. A rate matching is performed on the encoded data in module 103.
At present, an existed problem is that the adopted CRC generation polynomial is not optimal. Suppose the length of the load information for the PDCCH is A and L is the length of the check-bit sequence. Then the CRC corresponds to a linear block code (A+L, A). One technical index of the CRC generation polynomial is Pue. Pue can be defined as a probability that a linear block codeword is detected by error as another codeword after the channel transmission. A Binary Symmetric Channel (BSC) is taken as an example in following description.
The performance of gCRC16(D)=D16+D12+D5+1 is illustrated in FIG. 2. In this figure, x-axis indicates an error probability (ε) in BSC, y-axis indicates a corresponding Pue. The four curves in this figure corresponds to the cases A=16, 24, 32 and 48 respectively. From this figure, it is obviously seen: when ε is within the range [0.05, 0.3], Pue is even greater than that when ε=0.5. This means that the performance of the generation polynomial is very poor within the range.